Add exercise editor group
Add exercise editor group

This makes it easier to give exercise editing permissions to users that
shouldn't belong to the 'admin' group, for example trainers or similar.

kjagoo/wger_stark,rolandgeider/wger,petervanderdoes/wger,rolandgeider/wger,kjagoo/wger_stark,DeveloperMal/wger,petervanderdoes/wger,kjagoo/wger_stark,wger-project/wger,DeveloperMal/wger,DeveloperMal/wger,DeveloperMal/wger,wger-project/wger,rolandgeider/wger,petervanderdoes/wger,kjagoo/wger_stark,wger-project/wger,petervanderdoes/wger,wger-project/wger,rolandgeider/wger